Frequency Modulation, FM Modulation Index & Deviation Ratio

- notes and details of frequency modulation, FM, modulation index and deviation ratio.

Frequency modulation, FM tutorial includes:

In just the same way that it is useful to know the modulation index of an amplitude modulated signal the same is true for a frequency modulated signal. The modulation index is equal to the ratio of the frequency deviation to the modulating frequency. The modulation index will vary according to the frequency that is modulating the transmitted carrier and the amount of deviation. However when designing a system it is important to know the maximum permissible values. This is given by the deviation ratio and is obtained by inserting the maximum values into the formula for the modulation index.

This may also be expressed as:

For a VHF FM sound broadcast transmitter the maximum deviation is 75 kHz and the maximum modulation frequency is 15 kHz giving a deviation ratio of 5.

Modulation index and narrowband FM

narrow band FM is defined as the where the value of Β is small enough that the terms in the Bessel expansion, i.e. sidebands are negligible.

Often a figure of < 0.2 may be used.

Modulation index and wideband FM

Wideband FM is defined as the situation where the modulation index is above 0.5. Under these circumstances the sidebands beyond the first two terms are not insignificant.

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